She will go up against the country's current prime minister Prayuth Chan-o-cha who also announced that he would be running today.
It is the first time in recent Thai political history that an immediate member of the Royal Family has taken part in parliamentary politics but Ubolratana has shown a strong connection with the Shinawatra clan for a long time.
Thai Raksa Chart party leader Preechapol Pongpanich, holds up application of candidate for Prime Minister, Thailand's Princess Ubolratana Rajakanya Sirivadhana Barnavadi, at the election commission office in Bangkok, Thailand February 8, 2019.
She married American Peter Ladd Jensend in 1972, a decision which required her to give up her royal status.
Thailand has not had a royal as premier since becoming a constitutional monarchy in 1932.
Political analysts see the shocking move as a further political complication in the South-East Asian country ahead of its first election since the 2014 coup and return to democracy.
Arriving at the Election Commission this afternoon, the head of the pro-junta People's Reform Party asked the commission to reject the newly declared candidacy of Ubolratana Mahidol.
Since ascending the throne in 2016 King Maha Vajiralongkorn has reorganized palace affairs. Though Thaksin lives in exile, Thai Raksa Chart, which nominated Ubolratana, is one of several proxies for his influence.
"I have thoroughly considered the party's policies and I deem that they can further the programmes the [military regime] has initiated.so I accept the Phalang Pracharat Party's nomination as a prime minister candidate", Prayut said in a statement. "I want to reassure that I do not intend to hang on to power".
To paint this as an "upset" move in the last day of nominations is an understatement.
Ubolratana, the eldest child of King Bhumibol Adulyadej, was born in Lausanne in 1951. She returned to Thailand in the late 1990s after getting a divorce.
But she would not be covered by Thailand's strict lese majeste law against insulting the monarchy.
On her Instagram account, the princess recently posted a video of her eating street food and another complaining about Bangkok's pollution.
"I have been bored with politics for so long, but the princess has made me happy about the election again", one fan wrote on the web forum Pantip.